This SX-101a is my first HAM-only receiver, and has some features new
to me such as a product detector. I have spent many hours recapping
and aligning it, and pretty much all is well, but I have questions
about what is "normal" for this unit:

For AM reception you must listen to either the upper or lower
sideband, unlike my SX-71. I notice that no matter where the
selectivity is set (2,3 or 5 khz) that I must retune when switching
between sidebands. It is like the spread of the side band oscillator
crystals is a bit wider that the selectivity. I must tune lower up a
bit or upper down. Is this normal? If not, how can I close this gap a
bit?

I notice that the S/N ratio seems poor. There is always a lot of
background rush (kinda like white noise) compared to the SX-71. The
alignment process makes no reference as to what is normal. Even with
antenna terminals shorted there is still some noise, not
insignificant. How can I measure to see if this is within spec, and if
not, what might be the culprits?

I could not get 15 meter band to align... no matter how slug and
trimmer were adjusted I could not get the full bandspread as shown on
the dial. All other bands were fine. What might I look for to correct
this? I set 21.3mhz right on but both ends come up short of dial
setting.

BTW, for now my antenna is a 10ft wire, but that is what I was using
on the SX-71 which is my only comparison reference. Perhaps the 101
needs more antenna to sound good??? Any and all advice appreciated as
always.

Dan

geojunkie ) writes:

For AM reception you must listen to either the upper or lower
sideband, unlike my SX-71. I notice that no matter where the
selectivity is set (2,3 or 5 khz) that I must retune when switching
between sidebands. It is like the spread of the side band oscillator
crystals is a bit wider that the selectivity. I must tune lower up a
bit or upper down. Is this normal? If not, how can I close this gap a
bit?

I don't know about that particular receiver, but keep in mind that
except for one or two ham receivers (and some expensive professional
receivers), the BFO always moves when changing sidebands. There is
only one filter so the BFO has to move from one side of that filter
to the other.
-------
| |
| |
| |
BFO BFO
goes goes
here for here for
lower SB upper SB

The BFO is places so the suppressed carrier lies on the slope of
the filter, and the whole of the sideband fits into the filter with
the unwanted sideband being out of the picture.

The exceptions I mention have two filters, one for upper sideband and the
other for lower sideband, and the BFO is the common point between them:

------- -------
| LSB | | USB |
| | | |
| | | |
|
|
BFO
goes
here.

In this case, it works the same, but since there are two filters
the BFO stays on the same frequency while the filters are switched.
Not too common, since it's costly to have that extra filter.

Anyway, when you shift the BFO in the first case, you need to
retune to get the "suppressed carrier" of the incoming signal to
that spot. I'm too lazy at the moment to come up with some examples,
but think about the incoming signal. You get a certain beat note
on that signal. But if you shift the BFO, it and the incoming signal
can no longer cause the same beat note. You are retuning so the incoming
signal is once again on that curve of the filter, on the same side as
the BFO is now at.

That was the way of the early SSB receivers. With time, tricks came
into design play to compensate. One involves an additional conversion,
and I'm too lazy to try to explain it here. Besides, the most common
scheme is simply to add a bit of shift to the local oscillator so
the retuning is done automatically. So with any recent receiver, say
the past thirty years or so (at least decent receivers) one will not
notice the effect you mention. Even with older receivers this gimmick
was in place, but I'm not sure at what point one could say it was common.

Michael VE2BVW

"The exceptions I mention have two filters, one for upper sideband and the
other for lower sideband, and the BFO is the common point between them:

------- -------
| LSB | | USB |
| | | |
| | | |
|
|
BFO
goes
here.

In this case, it works the same, but since there are two filters
the BFO stays on the same frequency while the filters are switched.
Not too common, since it's costly to have that extra filter."

The SX-101a has two crystal controlled oscillators (selectable one at
a time for upper or lower sideband) for mixing the first IF signal
down to the second IF frequency. The BFO is deactivated during AM
mode. So what I am saying is that even at the widest selectivity
setting, switching between sidebands on AM mode requires retuning, as
if the two crystals have too wide a gap between them. I am going to
try and check them, but the output is probably way too low for my
counter (needs 100mv min).

Have you checked screen voltages, if any are low, a screen resistor has gone
up in value and the radio will sound weak!
hank wd5jfr
"geojunkie" wrote in message
om...
This SX-101a is my first HAM-only receiver, and has some features new
to me such as a product detector. I have spent many hours recapping
and aligning it, and pretty much all is well, but I have questions
about what is "normal" for this unit:

For AM reception you must listen to either the upper or lower
sideband, unlike my SX-71. I notice that no matter where the
selectivity is set (2,3 or 5 khz) that I must retune when switching
between sidebands. It is like the spread of the side band oscillator
crystals is a bit wider that the selectivity. I must tune lower up a
bit or upper down. Is this normal? If not, how can I close this gap a
bit?

I notice that the S/N ratio seems poor. There is always a lot of
background rush (kinda like white noise) compared to the SX-71. The
alignment process makes no reference as to what is normal. Even with
antenna terminals shorted there is still some noise, not
insignificant. How can I measure to see if this is within spec, and if
not, what might be the culprits?

I could not get 15 meter band to align... no matter how slug and
trimmer were adjusted I could not get the full bandspread as shown on
the dial. All other bands were fine. What might I look for to correct
this? I set 21.3mhz right on but both ends come up short of dial
setting.

BTW, for now my antenna is a 10ft wire, but that is what I was using
on the SX-71 which is my only comparison reference. Perhaps the 101
needs more antenna to sound good??? Any and all advice appreciated as
always.

Dan

"geojunkie" wrote in message
om...
This SX-101a is my first HAM-only receiver, and has some features new
to me such as a product detector. I have spent many hours recapping
and aligning it, and pretty much all is well, but I have questions
about what is "normal" for this unit:

For AM reception you must listen to either the upper or lower
sideband, unlike my SX-71. I notice that no matter where the
selectivity is set (2,3 or 5 khz) that I must retune when switching
between sidebands. It is like the spread of the side band oscillator
crystals is a bit wider that the selectivity. I must tune lower up a
bit or upper down. Is this normal? If not, how can I close this gap a
bit?

I notice that the S/N ratio seems poor. There is always a lot of
background rush (kinda like white noise) compared to the SX-71. The
alignment process makes no reference as to what is normal. Even with
antenna terminals shorted there is still some noise, not
insignificant. How can I measure to see if this is within spec, and if
not, what might be the culprits?

Do the signals really jump out of the backround noiise, or does it seem
like the radio is half deaf? I don't know the SX-101, but in general, a
very sensitive radio might very well have a significant amount of
backround noise. Of course, if the sensitivity seems low, you will have
to do some basic troubleshooting. The RF amp and the mixer are the main
suspects.

Be sure you didn't miswire anything when you recapped it. I'd like to
say such things never happen, but I miswired a ratio detector after I
fixed a bad connection at it's internal mica cap a couple of days ago.



I could not get 15 meter band to align... no matter how slug and
trimmer were adjusted I could not get the full bandspread as shown on
the dial. All other bands were fine. What might I look for to correct
this? I set 21.3mhz right on but both ends come up short of dial
setting.

The oscillator may be on the wrong side of the signal. If it's above
the signal, try setting it below the signal. Or vice versa. Don't
expect the oscillator to be on the same side of the signal as the other
bands.

BTW, for now my antenna is a 10ft wire, but that is what I was using
on the SX-71 which is my only comparison reference. Perhaps the 101
needs more antenna to sound good??? Any and all advice appreciated as
always.

Dan

Again, I don't know the 101, but I'd expect it to be considerably more
more sensitive than the run of the mill Hallicrafters radios such as the
S40A, which will start to work with a clip lead on the antenna terminal.

Frank Dresser

"Again, I don't know the 101, but I'd expect it to be considerably more
more sensitive than the run of the mill Hallicrafters radios such as the
S40A, which will start to work with a clip lead on the antenna terminal."

I did quite a bit of A/B listening between the two on Sunday. The
SX-71 seems 3-5db quieter. The fidelity on AM also seems better,
perhaps because it uses both sidebands? The Sx-101 is clearly superior
on SSB, although if you tweak and tune the Sx-71 does pretty well
there also. The 101 is far easier to discriminate adjacent channels
with in the crowded areas of the band and it has tons of gain left at
the same volume settings. However, I don't think the gain is useable
because the noise floor goes up with the gain settings. The 71 is
double conversion also, so either it must not be too shabby or I don't
have the 101 running right yet.